Photovoltaic device with transparent tunnel junction

ABSTRACT

A photovoltaic device includes a substrate, a semiconductor stack and a transparent tunnel junction. The semiconductor stack includes an n-type layer selected from a first transparent conductive oxide layer, or a window layer, or both; and a p-type absorber layer disposed on the n-type layer, wherein the absorber layer consists essentially of CdSexTe(1-x), wherein x is from 1 to about 40 at. %. The transparent tunnel junction comprises a transparent interface layer of Cd y Zn (1-y) Te doped to be p+type, and a transparent contact layer doped to be n+type, and the interface layer is disposed between the p-type absorber layer and the transparent contact layer. In bifacial embodiments, the tunnel junction forms a transparent back contact and electrode; and in multi-junction embodiments, the tunnel junction forms a diode-like connector between top and bottom cells. The transparent contact layer may comprise tin oxide or zinc oxide doped with aluminum, fluorine or indium. The photovoltaic device may also include an electron reflector layer and/or an optical reflector layer.

BACKGROUND

A photovoltaic device generates electrical power by converting lightinto direct current electricity using semiconductor materials thatexhibit the photovoltaic effect. The photovoltaic effect generateselectrical power upon exposure to light as photons are absorbed withinthe semiconductor material to excite electrons to a higher energy state.These excited electrons are thus able to move within the material,thereby causing current.

A basic unit of photovoltaic (PV) device structure, commonly called acell, may generate only small scale electrical power. Thus, multiplecells may be electrically connected together in series or parallel toaggregate the total power generated among the multiple cells within alarger integrated device, called a module or panel. A photovoltaic cellmay further comprise a protective back layer and encapsulant materialsto protect the cells from environmental factors. Multiple photovoltaicmodules or panels can be assembled together in series or parallel tocreate a photovoltaic system, or array, capable of generatingsignificant electrical power up to levels comparable to other types ofutility-scale power plants. In addition to photovoltaic modules, autility-scale array would further include mounting structures,electrical equipment including inverters, transformers, and othercontrol systems. Considering the various levels of device, fromindividual cell to utility-scale arrays containing a multitude ofmodules, all such implementations of the photovoltaic effect may containone or more photovoltaic devices to accomplish the energy conversion.

Thin film photovoltaic devices are typically made of various layers ofdifferent materials, each serving a different function, formed on asubstrate. A thin film photovoltaic device includes a front electrodeand a back electrode to provide electrical access to the photoactivesemiconductor layer or to other layers that are sandwichedthere-between.

Conventional thin film solar cells have modest conversion efficienciesfor converting light into direct current electricity. Thus, a key areain the field of PV devices is the improvement of the conversionefficiency.

One challenge to achieving higher efficiency is the difficulty increating a satisfactory back contact for ohmic contact and chargecarrier transport. Another challenge is the reduction of back surfacehole-electron recombination. A further challenge is improving thecollection of light. This includes collecting reflected light andcollecting light over a broad spectrum, including harvesting moreinfrared (IR) photons. Collection of charge carriers from both visiblelight and IR using conventional methods is impaired and inefficient,because the absorber layer thickness needed for IR absorption leads tominimal charge carrier creation or to charge carrier creation outsidethe depletion region. Diffuse light and light reflected off the groundand off nearby structures is available light that goes uncollected byconventional PV devices because it is reflected away by the rear of asolar panel, or is converted to heat.

Despite many improvements developed for ever increasing conversionefficiencies, there remains a continuing need for improved thin film PVdevices that minimize absorption losses while capturing or recoveringthe maximum amount of solar radiation practicable.

DRAWINGS

FIG. 1 depicts a schematic of functional layers in a first embodimentphotovoltaic device.

FIG. 2 is a bottom view of the photovoltaic device of FIG. 1 with theaddition of metallic grid fingers.

FIG. 3 depicts QE measurements for front-side (SS) and back-side (FS)illumination for an exemplary bifacial embodiment of FIG. 1.

FIG. 4 provides I-V curves for an exemplary bifacial embodiment of FIG.1.

FIG. 5 depicts a schematic of functional layers in a second embodimentof a photovoltaic device.

FIG. 6 is a QE comparison between a molybdenum nitride (MoNx/Mo) bilayerback contact and an exemplary embodiment of FIG. 5 with an AZO/Au backcontact.

FIG. 7A shows an I-V curve of the performance of an exemplary embodimentof FIG. 5 with an AZO/Au bilayer back contact of thickness 20 nm.

FIG. 7B shows an curve of the performance of an exemplary embodiment ofFIG. 5 with an AZO/Au bilayer back contact of thickness 150 nm.

FIG. 8 depicts a schematic of functional layers in a third embodiment ofa photovoltaic device.

FIG. 9 depicts a schematic of functional layers in a multi-junctionembodiment.

FIG. 10 shows the efficiency of several variations of transparent backcontacts normalized to the standard of MoNx/Al.

DETAILED DESCRIPTION

Provided are structures and compositions for use in photovoltaic (PV)devices. Embodiments provide thin film photovoltaic devices having ann-type transparent back contact layer. Embodiments include thin filmlayers comprising a tunnel junction, an optical reflector, and/or anelectron reflector.

The detailed description provided below in connection with the appendeddrawings is intended as a description of examples and is not intended torepresent the only forms in which the examples may be constructed orutilized. The description sets forth the functions of the examples andthe sequence of steps for operating with the examples. However, the sameor equivalent functions and sequences may be accomplished by equivalentalternative examples.

The following detailed description is not intended to limit theinvention, or the application and uses of the invention. As used herein,the word “exemplary” means “serving as an example, instance, orillustration.” Thus, any embodiment described herein as “exemplary” isnot necessarily to be construed as preferred or advantageous over otherembodiments. All of the embodiments described herein are provided toenable persons skilled in the art to make or use the invention and notto limit the scope of the invention which is defined by the claims.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary, or the following detailed description.

In this document, relational terms such as first and second, and thelike may be used solely to distinguish one entity or action from anotherentity or action without necessarily requiring or implying any actualsuch relationship or order between such entities or actions. Numericalordinals such as “first,” “second,” “third,” etc. simply denotedifferent singles of a plurality and do not imply any order or sequenceunless specifically defined. The sequence of the text in any of theclaims does not imply that process steps must be performed in a temporalor logical order according to such sequence unless it is specificallydefined by the language of the claim. The process steps may beinterchanged in any order without departing from the scope of theinvention as long as such an interchange does not contradict the claimlanguage and is not logically inconsistent.

In the following specification and the claims, the singular forms “a”,“an” and “the” include plural referents unless the context clearlydictates otherwise. As used herein, the term “or” is not meant to beexclusive and refers to at least one of the referenced components beingpresent and includes instances in which a combination of the referencedcomponents may be present, unless the context clearly dictatesotherwise.

Each of the layers described in the following embodiments may becomposed of more than one layer or film. Each layer can cover all or aportion of the PV device and/or all or a portion of the layer ormaterial underlying the layer. For example, a “layer” can mean any,amount of material that contacts all or a portion of a surface. During aprocess to form one of the layers, the created layer forms on an outersurface, typically a top surface, of a substrate or substrate structure.The substrate may include a base layer introduced into a depositionprocess and any other or additional layers that may have been depositedonto the base layer in a prior deposition process or processes. Layersmay be deposited over the entirety of a substrate with certain portionsof the material later removed through laser ablation, scribing, or othermaterial-removal process.

The manufacturing of a photovoltaic device generally includessequentially, disposing the functional layers or layer precursors in thestack through one or more processes, including, but not limited to,sputtering, spray, evaporation, molecular beam deposition, pyrolysis,closed space sublimation (CSS), pulse laser deposition (PLD), chemicalvapor deposition (CVD), electrochemical deposition (ECD), atomic layerdeposition (ALD), or vapor transport deposition (VTD).

Once a layer is formed it may be desirable to modify the physicalcharacteristics of the layer through subsequent treatment processes. Forexample, a treatment process step may include passivation, which isdefect repair of the crystalline grain structure, and may furtherinclude annealing imperfections or defects in the crystalline grain ofthe material that disrupt the periodic structure in the layer and cancreate areas of high resistance or undesirable current pathways thatare, for example, parallel to but separated from the desired currentpathway such as a shunt path or short.

An activation process may accomplish passivation through theintroduction of a chemical dopant to the semiconductor layer stack as abathing solution, spray, or vapor. Subsequently annealing the layer inthe presence of the chemical dopant at an elevated temperature promotesgrain growth and incorporation of the dopant into the layer. For manymaterials, a larger grain size, with fewer grain boundaries, reduces theresistivity of the layer, thereby allowing charge carriers to flow moreefficiently. The incorporation of a chemical dopant may also make theregions of the semiconductor layer more n-type or more p-type and ableto generate higher quantities of mobile charge carriers. Each of thesefeatures improves efficiency by increasing the maximum voltage thedevice can produce and reducing unwanted electrically-conductiveregions. In the activation process, the process parameters of annealtemperature, chemical bath composition, and soak time for a particularlayer depend on that layer's material.

The power output of a PV device is the product of the current (I, orsometimes J) and voltage (V) which can be shown by an I-V curve. At zerocurrent or “open circuit,” a maximum voltage is produced (V_(OC)) and atzero voltage or “short circuit,” a maximum current is produced (I_(SC)).The product of these is the total potential power (P_(T)), given inwatts (W), but this is not achievable in reality. The maximum poweroutput (P_(MAX)) achievable is defined by the point on the IV curve thatgives the largest product, I_(MP)*V_(MP). Fill Factor (FF) is defined asthe ratio of P_(MAX) to P_(T), i.e. the product of I_(MP)*V_(MP) dividedby the product of I_(SC)*V_(OC). Higher FF is indicative of a moreefficient cell. The conversion efficiency of a PV device is the ratio ofthe total potential power (P_(T)) discounted by FF, over the totalincident power (P_(in)), and may be represented mathematically as:Efficiency=(I_(SC)*V_(OC)*FF)/P_(in). One objective in the field ofphotovoltaic devices is the improvement of conversion efficiency.Increasing the sulfur or selenium content within an absorber comprisingCdTe can alter the band gap energy. Increasing the selenium content from0 atomic percent (at. %), to about 40 at. % compared to tellurium withinan absorber consisting primarily of CdTe decreases the band gap energy,which improves infra-red absorption and can thereby increase currentproduction.

Grading can be used to tune the bandgaps of CdTe based alloys to reducesurface recombination and increase absorption of the solar spectrum toimprove the power conversion efficiency. The gradient may be formed byeither depositing a material(s) having a desired gradient and materialprofile, or the gradient may be formed by depositing discrete layers ofmaterial that are subsequently annealed to create a desiredconcentration profile. Each layer can be graded as the layer moves awayfrom the p-n metallurgical junction in order to optimize band-gapalignment and/or doping at the side in contact with the high dopedcontact layers.

The addition of one or more barrier layers or buffer layers may be usedto inhibit diffusion of dopants or contaminants between layers. A bufferlayer may be utilized to reduce the number of irregularities arisingduring the formation of a semiconductor.

In the present disclosure, when a layer is being described as beingdisposed or positioned “on” another layer or substrate, it is to beunderstood that the layers can either be directly contacting each otheror have one (or more) layer or feature between the layers. Further, theterm “on” describes the relative position of the layers to each otherand does not necessarily mean “on top of” since the relative positionabove or below depends upon the orientation of the device to the viewer.Moreover, the use of “top,” “bottom,” “above,” “below,” and variationsof these terms is made for convenience, and does not require any,particular orientation of the components unless otherwise stated.However, the orientation remains consistent within each embodiment orexample, such that if B is “on” A, and C is “on” B, then B isnecessarily between A and C, though not necessarily in contact witheither.

In the present disclosure, when an object is being described as being“adjacent,” it is to be understood that the word adjacent means “nextto” and “in direct contact with” another object and is not synonymouswith the term “on,” although one object can be “on” and “adjacent” toanother object.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about”, “approximately,” and “substantially” is notto be limited to the precise value specified. In some instances, theapproximating language may correspond to the precision of an instrumentfor measuring the value. Here and throughout the specification andclaims, range limitations may be combined and/or interchanged, suchranges are identified and include all the sub-ranges contained thereinunless context or language indicates otherwise.

The term “transparent layer” as used herein, refers to a layer thatallows an average transmission of at least 70% of incidentelectromagnetic radiation having a wavelength in a range from about 300nm to about 900 nm. Radiation in this wavelength range is considered“light” for purposes of this invention, even though it may not beentirely in the visible range.

The term “absorber layer” as used herein refers to a semiconductinglayer wherein the absorption of electromagnetic radiation causeselectrons in the absorber layer to be excited from a lower energy“ground state” or “valence band” in which they are bound to specificatoms in the solid, to a higher “excited state,” or “conduction band” inwhich they can move about within the solid.

As used herein, the “effective carrier density” refers to the averageconcentration of holes and electrons in a material.

As used herein, “n-type layer” refers to a semiconductor layer having anexcess of electron donors as majority carriers; while a “p-type layer”refers to a semiconductor layer having an excess of electron acceptors(also known as “holes”) as majority carriers. In each case the excesscarriers (electrons or holes) may be provided by chemically doping thesemiconductor with suitable dopants or may be generated by intrinsicdefects present in the material. N-type layers and p-type layers thatare chemically doped can have other materials in addition to n-type andp-type dopants. For example, a p-type layer of CdSeTe is a layer formedof Cd, Se, and Te that is also chemically doped p-type. For junctionpartners in PV device, n-type and p-type layers or materials interfacedtogether, adjacent one another. Generally, either the p-type material orlayer or the n-type material or layer will serve as the “absorber layer”where electrons are excited as described above to generate thephotovoltaic effect.

Semiconductors doped to be p-type or n-type are sometimes furthercharacterized based on the density of respective majority chargecarriers. Although the boundaries are not rigid, a material is generallyconsidered p-type if electron acceptor carriers (i.e. “holes”) arepresent in the range of about 1×10¹¹ cm⁻³ to about 1×10¹⁶ cm⁻³, andp+type if acceptor carrier density is greater than about 1×10¹⁶ cm⁻³.Similarly, a material is considered n-type if electron donor carriersare present in the range of about 1×10¹¹ cm⁻³ to about 1×10¹⁶ cm⁻³, andn+type if donor carrier density is greater than about 1×10¹⁶ cm⁻³. Theboundaries are not rigid and may overlap because a layer may be p+relative to a layer that is p-type (or n+ relative to a layer that isn-type) if the carrier concentration is at least 2 orders of magnitude(i.e. 100-fold) higher, regardless of the absolute carrier density.Additionally, some consider charge densities of greater than about1×10¹⁸ cm⁻³ to be “++” type; and thus a layer of either n-type or p-typecan be “++” relative to a layer of the same type that is itself “+”relative to yet a third layer, if the ++ layer has a same-type carrierdensity more than 100 fold that of the + layer.

While exemplary embodiments are presented, it should be appreciated thata vast number of variations exist. It should also be appreciated thatthe exemplary embodiment or exemplary embodiments are only examples, andare not intended to limit the scope, applicability, or configuration ofthe invention in any way. Rather, the detailed description providesthose skilled in the art with a road map for implementing exemplaryembodiments of the invention. It is to be understood that variouschanges may be made in the function and arrangement of elementsdescribed in an exemplary embodiment without departing from the scope ofthe invention as set forth in the appended claims.

In the following description, references are made to the accompanyingdrawings that form a part hereof, and which are shown by way ofillustration, specific embodiments, or examples. Referring now to thedrawings, in which like numerals represent like elements through theseveral figures, aspects of a photovoltaic device will be described.

The formation of stable, low resistance back contacts to CdTe solarcells can be challenging because typical metals used for back contactsto CdTe do not have work functions large enough to make good ohmiccontacts to CdTe, and tend to form Schottky, or blocking barriers. Atypical approach to overcome this obstacle is to either reduce thebarrier or moderate its width by heavily doping the CdTe with Cu at theCdTe-back contact interface. Doing so may improve initial deviceperformance, however, the diffusion of copper atoms from theback-contact region towards the heterojunction over time leads todecreased Voc and contributes to device degradation. Back-contactdegradation typically results in reduced fill factor (FF), due to anincrease in series resistance and/or the formation of a blocking backdiode.

Other challenges to achieving higher efficiency are the difficulty increating a satisfactory ohmic back contact for reducing back surfaceminority electron recombination, and maximal utilization of allwavelengths of available light. To collect more IR light, conventionaltechniques dictate using thicker absorber layers. However, since IRlight penetrates too deeply, even for thick absorption layers, theefficient collection of charge carriers generated from these wavelengthsis not feasible because the charge carriers are created outside thejunction depletion region.

One way to increase collection of IR radiation, in conjunction withvisible light, is to use a multi junction device. Multi-junction solarcells can achieve higher total conversion efficiency than singlejunction cells by capturing a larger portion of the solar spectrum.These devices can be formed as monolithically integrated structures withmore than one p-n junction and with materials having different band-gapproperties responsive to different ranges of the spectrum. For a devicewhere the primary light source is from above, a light-incident upon theuppermost cell has a large band gap to capture energetic shortwavelengths, while a lower cell has a smaller band gap and captureslonger wavelengths and reflected photons. A multi-junction device mayhave two (tandem) or more sub-cells with tunnel junctions between thesub-cells.

Tunnel junctions serve a variety of different purposes. In photovoltaiccells, tunnel junctions form connections between successive p-njunctions, by connecting the n terminal of a first diode with a pterminal of a second diode or vice versa. Tunnel junction layersfunction as an ohmic electrical contact within a photovoltaic device.

There are challenges in using CdTe as a tunnel junction material in amulti junction device. One reason is because CdTe is difficult toeffectively dope p+, which makes the creation of a tunnel junction in aCdTe layer problematic for some applications, such as a tandem cell. Onesolution to this challenge is to selectively match materials suitablefor use with CdTe that can also be tuned by compositional variation tobe p+ to produce a tandem cell with good absorption characteristics.

Diffuse light and light reflected off the ground and off nearbystructures is available light that goes uncollected because it isreflected away by the rear of a solar panel, or is converted to heat.One solution to this light collection challenge is to make adouble-sided, or bifacial device. Light may then be collected throughboth the primary or front surface as well as the secondary or back-sidesurface of a photovoltaic device into an interior absorber layer forconversion to current. Bifacial CdTe-based thin film devices havehistorically shown low conversion enhancement of the back sideillumination, however, because the layer configuration causesrear-side-absorbed photons to be generated some distance from theheterojunction. One solution to this problem is to make a bifacialdevice with an electron reflector layer.

In bifacial embodiments, a plurality of thin-film semiconductor layers(a “stack”) are sandwiched between the front side surface and the backside surface, the front side being the “sunny” side facing the solarradiation. Within the semiconductor stack, an outermost layer on thefront side is an n-type transparent conductive oxide; an outermost layeron the back side is a secondary transparent conductive oxide, which maybe p-type or n-type. Although a p-type transparent conductive layerwould be preferred as a contact to the p-type absorber or interfacelayer, they are difficult to fabricate. An n-type transparent conductivelayer is easier to fabricate, but requires a tunnel junction to contactthe p-type absorber or p-type interfacial layer. A p+type interfacelayer is adjacent the n+type transparent conductive oxide; and a p-typeabsorber layer is between the p+type interface layer and the n-typetransparent conductive oxide. Of course, there may also be substrates,superstrates, encapsulation layers, antireflection layers and/or otheroptional layers outside of (i.e. in front of) the front side surface oroutside of (i.e. behind) the back side surface.

A photovoltaic device may include an electron reflector (ERF) layer toimprove the flow of electrical current by providing an ohmic contact toachieve high performance efficiency, at the interface between theabsorber layer and an interface layer or back contact layer. An electronreflector reduces charge loss between the absorber layer and the backcontact by reducing the recombination of electron-hole pairs at thesurface of the absorber layer closest to the back contact.

An ERF layer between a cadmium telluride (CdTe) or cadmium selenidetelluride absorber layer and a back current pathway may be formed from amaterial which has a higher band gap than the absorber. The higher bandgap ERF layer then provides a conduction-band energy barrier whichrequires higher energies for electron movement, thereby reducing thenumber of electrons having the tendency and energy to migrate across theERF layer into the back contact where they may potentially recombinewith holes flowing out the back contact.

An ERF layer may include a layer of Zinc Telluride (ZnTe), or ManganeseTelluride (MnTe) or Magnesium Telluride (MgTe). Alternatively anelectron reflector layer may include a layer of ternary compound such asCadmium Zinc Telluride (CdZnTe), Cadmium Manganese Telluride (CdMnTe) orCadmium Magnesium Telluride (CdMgTe), or the combinations of the binaryand ternary compounds or other materials that have appropriate band gapstructure. These and similar compounds may be characterized genericallyby the formula Cd_(z)M_((1-z))Te, where M represents Zn, Mg or Mn, and zis from 0 to about 99 at. %, typically from 0 to about 60 at. %.

In some structures, an ERF may include a dopant. In some embodiments anERF comprises a layer of ZnTe doped with Copper Telluride (Cu₂Te) up toa 5% concentration on an atomic basis. Alternatively, an electronreflector layer may include a hi-layer of materials having differentcompositions, with a first layer including ZnTe or cadmium zinctelluride (CZT or CdZnTe) and a second layer including Cu₂Te. In someembodiments, an electron reflector may be between 5 nm to 25 nm thick orbetween about 15 nm to 20 nm thick.

In some embodiments, an ERF may have a dopant gradient through thethickness of the ERF. For example the concentration of Cu₂Te may be0.01% at an absorber/ERF junction increasing to 5% at the junctionbetween the ERF and a back electrode layer. The dopant gradient mayincrease stepwise or it may be continuously increasing. Electronreflectors are more fully discussed in co-owned U.S. Pat. No. 9,269,849,which is herein incorporated by reference in its entirety.

Turning now to the figures, FIG. 1 depicts an embodiment of aphotovoltaic device 100 according to the invention, wherein a number oflayers are shown and described. The layers described, the materialsused, and/or the methods of forming the layers of the photovoltaicdevice 100 may be substituted, included in addition to layers described,or be absent in the embodiments of the invention described herein belowand illustrated in the figures. It is further understood that each ofthe layers may be deposited in a single layer deposition from a singlematerial, from a multi-layer process from a single material, or from amulti-layer process from a plurality of materials.

The photovoltaic device 100 of FIG. 1 includes a substrate layer 110, afirst transparent conductive oxide (TCO) layer 120, a first p-typeabsorber layer 130, an interface layer 140, and a transparent contactlayer 150 wherein the transparent contact layer 150 serves as a backcontact. The photovoltaic device further includes electrical connectionsthat provide a current path to communicate generated current flow, suchas from one photovoltaic cell to adjacent cells in a module or from onephotovoltaic module to adjacent modules in an array. Alternatively, theelectrical connections may direct the current flow to an external loaddevice where the photo-generated current provides power. In anembodiment, the first TCO layer 120 serves as the front electrode wherea first conductive lead is attached and the transparent contact layer150 serves as the back electrode where a second conductive lead isattached.

The substrate layer 110 provides a surface upon which layers of materialare disposed to create the photovoltaic device. The substrate layer 110comprises any suitable substrate, such as soda lime glass, float glassor low-iron glass. Alternatively, the substrate layer 110 may includepolymeric, ceramic, or other materials that provide a suitable structurefor forming a base of a photovoltaic cell. The substrate is notessential to the invention, but is a practical medium for applying thesubsequent thin film layers. Although layers are generally, describedherein in a superstrate configuration, in which front side light isincident through the substrate, a true “substrate” configuration, inwhich the substrate is on the side of the back contact, is alsopossible.

The substrate layer 110 may have various surface coatings on theinternal and external surfaces, in the context of the finished device.The substrate layer 110 may have additional external layers applied thatimprove the transmission of light and device performance, which mayinclude anti-reflective coatings and/or anti-soil coatings. Thesubstrate may also have coatings on the internal surface, such as forexample, a buffer layer or a barrier layer. A barrier layer may be usedto promote the chemical stability of the substrate layer 110 byinhibiting diffusion of ions from, into, or across the substrate. Thebarrier layer may be formed of, for example, silicon nitride, siliconoxide, aluminum-doped silicon oxide, boron-doped silicon nitride,phosphorus-doped silicon nitride, silicon oxide-nitride, or combinationsor alloys thereof.

The first TCO layer 120 provides a transparent, electrically conductivematerial serving as a front electrode to the photovoltaic device tocommunicate a generated electrical current to a circuit, which mayinclude an adjacent photovoltaic device, such as to adjacent cellswithin a photovoltaic module. The internal coatings together with atleast one transparent conductive oxide layer comprise a TCO stack.

The first TCO layer 120 may be formed from a transparent conductiveoxide. Exemplary transparent conductive oxides include, but are notlimited to, tin oxide, zinc oxide, zinc sulfide, cadmium oxide andgallium oxide, which may be doped with elements like indium, fluorine,or other dopants; such as indium gallium oxide, indium tin oxide, indiumzinc oxide, cadmium stannate (Cd₂SnO₄), cadmium tin oxide, indium dopedcadmium oxide, fluorine doped tin oxide, aluminum doped zinc oxide,indium tin oxide, and combinations and doped variations thereof.

In the embodiment shown in FIG. 1, the TCO is doped to form an n-typesemiconducting layer. The first TCO layer 120 can be formed over thesubstrate layer 110, or a barrier layer (if included). In one particularembodiment, the first TCO layer 120 comprises a bilayer of a fluorinedoped tin oxide layer (SnO₂:F) with an undoped, SnO₂ layer.

The TCO stack may further include additional material layers appliedover first TCO layer 120 surface, such as a buffer layer that promotesthe electrical function of the TCO or that provides an improved surfacefor the subsequent deposition of semiconductor materials. In anembodiment, a barrier layer is formed over the first TCO layer andcomprises a material selected from: tin oxide, zinc tin oxide, zincoxide, zinc oxysulfide, or zinc magnesium oxide. In an embodiment, thefirst TCO layer 120 comprises fluorine doped tin oxide layer (SnO₂:F)having a carrier concentration of 1×10¹⁷ to 1×10¹⁹ cm⁻³, with anundoped, higher resistivity SnO₂ buffer layer deposited on the TCO layer120. The SnO₂ buffer layer has a thickness of about 20 nm to 100 nm.

In some embodiments an optional n-type window layer is disposed betweenthe first TCO layer 120 and the first absorber layer 130 to serve as ann-type semiconductor junction partner for a p-type absorber. Cadmiumsulfide and other suitable n-type window semiconductors may be used. Inother configurations, as shown in FIG. 1, window layers are optional andmay be omitted, in which case the TCO layer 120 may serve as the n-typejunction partner. Thin film devices without a conventional window layerare more fully discussed in co-owned U.S. Patent Pub. No. 2016/0126395A1, which is herein incorporated by reference in its entirety.

The first absorber layer 130 is disposed over the first TCO layer 120and the substrate layer 110. The first absorber layer 130 comprises ap-type semiconductor material to form a p-type region within thephotovoltaic device 100. In an embodiment, the first TCO layer 120 andthe first absorber layer 130 form a n-p junction. The first absorberlayer 130 absorbs photons passing through the first TCO layer 120 tomobilize charge carriers. The first absorber layer 130 may be ternaryalloy formed from cadmium telluride containing selenium (CdSeTe). UsingSe with CdTe in the first absorber layer 130 provides additional controlover bandgap (E_(g)) variation because the bandgap of the first absorberlayer 130 is altered by the presence of Se therein. For example, thebandgap of the first absorber layer 130 is decreased by increasing Secontent up to 40 at. % compared to Te. In an embodiment, the compositionof the first absorber layer 130 may be represented by the formulaCdSe_(x)Te_((1-x)) where x is between about 1 and about 40 at. % (i.e.,about 0.01 to about 0.40). If graded, the highest concentration can bebetween about 15 at. % and about 40 at. % (i.e., about 0.15 to about0.40) such as, for example, between about at. % 15 and 30 at. % (i.e.,about 0.15 to about 0.40). The first absorber layer 130 may be between0.5 μm to 8.0 μm thick, or between 1.0 μm to 3.5 μm thick, or between1.85 and 2.25 μm thick. In one exemplary embodiment the first absorberlayer 130 is 2.1 μm CdSe_(x)Te_((1-x)) where the average value of x isless than 25 at. %.

The first absorber layer 130 may be formed by a layer of material thatis deposited on the photovoltaic device 100, or the first absorber layer130 may be formed by a plurality of layers of material deposited on thephotovoltaic device 100 that are processed, such as by annealing, toform an alloy. A gradient may be formed within the first absorber layer130 represented by a continuous increase in concentration, a stepwisechange in concentration or the like. Such a gradient may apply to Se inthe case of a ternary alloy, wherein the value of “x” varies throughoutthe thickness of the absorber, or to a dopant in the absorber.

The interface layer is indicated at 140. The interface layer 140comprises zinc telluride or cadmium zinc telluride heavily doped p+ withcopper, silver, or gold, and may be represented by the formulaCd_(y)Zn_((1-y))Te:D, wherein y may be from 0 to about 90 at. %, forexample, from 0 to about 60 at. %, or from about 30 at. %, to about 60at. %, and “:D” indicates a dopant in the formula.

In the case of a tandem solar cell, the Cd_(y)Zn_((1-y))Te layer dopedwith Cu forms the p+layer of a tunnel junction connecting the first(“top”) and second (“bottom”) solar cells as shown in FIG. 9. In otherembodiments, the interface layer 140 comprises Cd_(y)Zn_((1-y))Te:Ddoped with a different noble metal (such silver or gold), a Group Vmaterial (such as nitrogen, phosphorous, or arsenic), or a combinationof materials from both groups. The interface layer 140 ranges inthickness from 10 nm to 50 nm. In one particular embodiment, theinterface layer 140 is Cu doped ZnTe with a thickness of 25 nm and acopper concentration of 0.01 to 1.0% Cu by atomic weight.

The transparent contact layer 150 must be transparent and exhibitsuitable electronic characteristics for a back contact to the firstabsorber layer 130 and the capability to serve as a n+layer of a tunneljunction. Among the desired electronic properties is that the workfunction is a good match to the electron affinity of the first absorberlayer 130 such that the positive charge carriers (holes) can flowreadily into the transparent contact layer 150. The transparent contactlayer 150 comprises a transparent conductive oxide (TCO) that is heavilydoped n+. Some suitable doped oxides include zinc oxide or tin oxidedoped with aluminum, indium, or fluorine, or cadmium stannate. OtherTCOs capable of being heavily doped n+ may also be suitable.

The transparent contact layer 150, in conjunction with the interfacelayer 140, functions as transparent tunnel junction. In bifacialembodiments, as shown in FIG. 1 and FIG. 8, the transparent contactlayer 150 allows light incident on a rear surface to pass through to theabsorber and serves as a back contact. In ORF embodiments, as shown inFIG. 5, the transparent contact layer 150, transmits lightbidirectionally: first from the absorber through to a reflective surfaceof the ORF, and second to pass reflected light back through layer 150 tothe absorber layer again. In a tandem device, as shown in FIG. 9, theheavily doped transparent contact layer 150 forms the n+ layer of atunnel junction connecting the first (“top”) cell 410 and second(“bottom”) cell 420.

Thus, the transparent contact layer 150 serves more than one purpose.First, the transparent contact layer 150 allows reflected light andback-side incident light to enter from the back of the photovoltaicdevice 100 to be captured by the first absorber layer 130 to generateelectrons and holes. Second, the transparent contact layer 150 is a n+tunnel junction layer allowing a more efficient transport of holes fromthe first absorber layer 130 to the back electrical connection byreducing inadvertent electron/hole recombination that occurs betweenconventional back contact and back electrode layers. Third, it providesa transparent ohmic contact for use in a multi-junction device.

In some embodiments, the transparent contact layer 150 is AZO whereinthe aluminum doping level is in a range from 2 at. % to 8 at. % and thethickness is in a range from 20 nm to 1000 nm. In an embodiment, thealuminum doping level is 5 at. % and the transparent back contact layer150 is about 500 nm thick.

In the embodiment of FIG. 1 and in all other embodiments describedherein, the transparent conductive layer 120 refers to an electricallyconductive layer that is transparent, which can serve as a frontelectrical contact. Similarly, the transparent contact layer 150 is alsoan electrically conductive layer that is transparent, and which can alsoserve as a back electrical contact in a bifacial or ERF embodiment.

FIG. 1 depicts an optically bifacial photovoltaic device 100. A bifacialphotovoltaic device 100 can collect light incident on a front surfacethrough the first TCO layer 120 and collect light incident on a rearsurface through the transparent contact layer 150. The performance ofthe photovoltaic device 100 can be improved by adjusting the thicknessesof layers, such as the first absorber layer 130 and the transparentcontact layer 150, passivating the back contact interface to reduceelectron-hole recombination, and/or utilizing a secondary conductor. Onesuch implementation is depicted in FIG. 2.

FIG. 2 shows the device of FIG. 1 with the addition of a secondaryconductor 180. The secondary conductor 180 may be a set of metallic gridfingers spaced apart to allow light to pass into the photovoltaic device100 from the rear through the transparent contact layer 150. Thesecondary conductor 180 can be used in combination with a thinnertransparent contact layer 150. In some embodiments, the device backcontact consists essentially of the transparent contact layer 150 andthe secondary conductor 180. In some embodiments, having a secondaryconductor 180, the transparent contact layer 150 is aluminum doped zincoxide wherein the aluminum doping level is about 5 to 6 at. %, with athickness less than 500 nm. In some embodiments the transparent contactlayer 150 thickness is between 100 nm and 300 nm.

In some embodiments, the secondary conductor 180 comprises parallelwires. In some embodiments, the secondary conductor 180 comprisesparallel metallic ribbons, wider in the cross-stack direction to carrymore current, and narrower in the cross-plane direction to minimizeshading and obscuration loss. In some embodiments, the secondaryconductor 180 composition comprises one or more materials selected from:Au, Cu, Al, and Ag. In some embodiments, the secondary conductor 180composition is substantially free of copper. In some embodiments, thesecondary conductor 180 is embedded, or partially embedded, in thetransparent contact layer 150. In some embodiments, the secondaryconductor 180 is adjacent to, and directly contacting, the transparentcontact layer 150. In some embodiments, the back side may furthercomprise a back surface layer or encapsulant. In some embodiments, thesecondary conductor 180 contacts and is adjacent to the transparentcontact layer 150 and is embedded, or partially embedded, in a backsurface layer or encapsulant.

In some embodiments, the composition of the back surface layer, thesecondary conductor 180, the transparent contact layer 150, and theinterface layer 140 are all substantially free of copper. In someembodiments, the composition of the back surface layer, the secondaryconductor 180, the transparent contact layer 150, and the interfacelayer 140 are all substantially free of graphite.

Quantum efficiency (QE) is a combination of the generation ofelectron-hole pairs and the effectiveness of carrier collection within asolar cell. QE is the ratio of charge carriers produced to the incidentphotons. A QE value indicates the amount of current that a photoelectricdevice will produce when irradiated by photons of a particularwavelength. The photoelectric device quantum efficiency (QE) may beintegrated over a solar spectral range, and the QE may be expressed as apercentage value. Therefore, the QE can be used in predicting the amountof current that a photovoltaic cell will produce when exposed tosunlight or other incident light.

FIG. 3 depicts Quantum Efficiency (QE) measurements for front-side (SS)and back-side (FS) illumination of a bifacial photovoltaic device havinga first (CdSeTe) absorber layer 130 2.1 μm thick and a 500 nm thickaluminum-doped zinc oxide (AZO) transparent contact layer 150. The QEmeasurements were taken by directly illuminating the front-side (SS) andthe back-side (FS) with an identical light source. As can be seen fromthe back-side (FS) illumination portion of the curve there is currentcollection occurring through the transparent contact layer 150 (here,AZO), particularly in the red and infrared end of the spectrum, withsignificant back-side IR light absorption between 800 and 900 nm. Theseresults demonstrate the ability to use AZO in a bifacial solar cellconfiguration and show good IR absorption. Back-side (FS) collection canbe further improved by adjusting the thicknesses of layers, such as thefirst absorber layer 130 and the transparent contact layer 150,passivating the back contact interface to reduce electron recombination,and/or utilizing a secondary conductor. FIG. 3 also shows that in thelimit of thin absorbers, back-side (FS) QE converges with front-side(SS) QE.

FIG. 4 provides an I-V curve showing the performance of the photovoltaicdevice, as well as the values for typical cell performance parameterssuch as current density (JQE), and fill factor (FF). As can be seen, thecurrent density is 27.8 mA and FF is 72%. The observed non-rectifyingI-V curve indicates that ohmic tunnel junctions can be formed with theuse of an AZO transparent contact layer 150. This allows for practicaltunnel junctions utilizing typical CdTe processes for the formation ofthe first p+ tunnel junction layer and cost-efficient manufacturingprocesses for the creation of the second n+ tunnel junction layer.

FIG. 5 is a depiction of an exemplary photovoltaic device 200. Thephotovoltaic device 200 of FIG. 5 is constructed similarly to that ofthe photovoltaic device of FIG. 1 with the addition optical reflectorlayer 160. The optical reflector (ORF) layer 160 is disposed on thetransparent contact layer 150 to improve the efficiency of thephotovoltaic cell 200. Some light wavelengths pass through thephotovoltaic cell 200 without being absorbed in the first absorber layer130. The purpose of the ORF layer 160 is to reflect this unabsorbedlight back into the first absorber layer 130 where the light gets asecond chance to be absorbed to increase carrier generation and therebyincrease efficiency. The optical reflector layer 160 comprises metallicAu, Ag, Al or other suitable material. In one exemplary embodiment, theORF layer 160 comprises a metallic Au layer in a thickness between 20 nmand 500 nm, typically between 50 and 200 nm, for example between 75 and150 nm. In some embodiments, the ORF may be deposited as a bilayer ofaluminum with a thin layer of either gold or silver as a second layer.

To obtain both good optical transmission and electrical contact in theembodiment of FIG. 5, the transparent contact layer 150 is disposedbetween the transparent interface layer 140 and the optical reflectorlayer (ORF) 160. Together, the transparent contact layer 150 and theoptical reflector layer 160 form a back contact. In one exemplaryembodiment, the transparent contact layer 150 is 5 at. % Al doped ZnOranging in thickness between 20 nm and 150 nm. Note this is distinctfrom the first TCO layer, which may optionally be a bilayer offluorine-doped SnO₂/SnO₂ as described above.

The curve of FIG. 6 is a QE comparison between a molybdenum nitride(MoNx/Mo) bilayer back contact and an exemplary embodiment of FIG. 5with an AZO/Au Mayer back contact. The QE curve shows efficiency gainsat long wavelength light realized by using an AZO/Au back contact ratherthan a metal oxide back contact such as molybdenum nitride (MoNx/Mo orMoNx/Al). As can be seen, there is enhanced current collection by theAZO/Au back contact at wavelengths between about 0.8-0.9 μm (800-900 nm)demonstrating a positive optical reflection effect from the AZO/Au backcontact. There is little value in adding a high reflective metal on MoNxbecause its high absorption compared to AZO obscures any, reflectivebenefit.

FIGS. 7A and 7B present I-V curves to compare the performance of anexemplary embodiment of FIG. 5 having an AZO/Au back contact with aparticular thickness. FIGS. 7A and 7B each provide the values fortypical cell performance parameters such as open circuit voltage (Voc),short circuit current density (here denoted Jsc), and fill factor (FF).As can be seen, there is little Jsc difference observed between a 20 nmand 150 nm ZnO:Al back contact. This result demonstrates that AZO layerthickness does not significantly affect current because of the goodtransparency of the AZO. In addition, comparing the I-V curves fromFIGS. 7A and 7B with FIG. 4 shows that the addition of the opticalreflector layer 160 to form an AZO/Au back contact performed well bygenerating more current and voltage than the device utilizing a 500 nmthick AZO transparent contact layer 150 without an optical reflectorlayer.

Other transparent contact layers performed equally well or better. FIG.10 compares the efficiencies of additional PV devices having an ORF 160as in FIG. 5, but having varying ORF/back contact compositions. In eachcase the experimental compositions were within a few percentage pointsof a 10 nm MoNx/Al back contact, which served as the control to whichthe data are normalized. The experimental ORF/back contacts werebilayers of a 20 nm metal oxide layer and a 120 nm aluminum metal layer.The oxide layers were cadmium stannate (Cd2SnO4), aluminum doped zincoxide (AZO), and indium doped tin oxide (ITO).

FIG. 8 is a depiction of an exemplary photovoltaic device 300. Thephotovoltaic device 300 is a bifacial embodiment similar to theembodiment depicted in FIG. 1 with the addition of an optional firstwindow layer 185 and an optional electron reflector (ERF) layer 170. Theoptional first window layer 185 is disposed on the first TCO layer 120and may be formed from a substantially transparent n-type semiconductormaterial such as, for example, CdS, CdSSe, CdSe, zinc sulfide (ZnS),ZnSe, ZnTe, a ZnS/CdS alloy, ZnSO, cadmium magnesium sulfide, or anothersuitable wide-band gap and stable material. The first window layer 185is disposed between the first transparent conductive oxide layer 120 andthe first absorber layer 130.

When utilizing the n-type first window layer 185, an optional highresistivity transparent (HRT) layer or buffer layer may be disposedbetween the first TCO layer 120 and first window layer 185 to promoteelectrical function of the TCO, such as preventing shunt defects orreducing the effect of pinholes or weak diodes, or to provide animproved surface for the subsequent deposition of semiconductormaterials. The HRT layer may be a bilayer utilizing SnO₂ plus anintrinsic SnO₂ layer between the low resistance SnO₂ and the firstwindow layer 185. Or, the HRT layer may be any one of the groupspecified for the first TCO layer 120 but without doping so that theelectrical resistance is high. In an embodiment, the HRT is ZnO or SnO₂with a thickness of about 25 nm to about 200 nm. In an embodiment, theHRT is ZnO or SnO₂ with a thickness from about 50 nm to about 100 nm.The buffer layer may be formed of, for example, tin oxide, zinc tinoxide, zinc oxide, zinc oxysulfide, or zinc magnesium. The photovoltaicdevice 200 may omit the first window layer 185, due to the presence ofthe n-type first TCO layer 120.

The ERF layer is deposited between the first absorber layer 130 and theinterface layer 140, which is part of the tunnel junction. The ERF layer170 may include CdZnTe (“CZT”), CdMnTe, CdMgTe, or similarly suitablematerials with a higher band gap than the CdSe_(x)Te_((1-x)) firstabsorber layer 130, as described above. In one embodiment, the ERF 170is a zinc telluride (ZnTe) layer doped with copper telluride (Cu₂Te). Inthis embodiment, the ERF 170 contains primarily ZnTe with a Cu₂Te dopantpresent in concentrations up to 5 at. %. The ERF 170 may be betweenabout 5 nm to about 25 nm thick.

The higher band gap ERF layer 170 provides a conduction-band energybarrier which requires higher energies for electron movement across it.This makes it more difficult for electrons to move out of the firstabsorber layer 130 toward the transparent contact layer 150 where theymay recombine with the concentration of holes, and instead, reflects theelectrons toward the first TCO layer 120, contributing to currentcollection and decreasing charge carrier recombination. Minimizingcrystal lattice defects, such as by passivation at the ZnO:Al layer alsocontributes to decreasing the concentration of recombination sites, andthereby increases rear side energy collection. In an embodiment, an ERFlayer 170 is used in a device with an optical reflector layer 160.

FIG. 9 shows one embodiment of a tandem photovoltaic device 400. Thephotovoltaic device 400 includes the elements of photovoltaic device 100from FIG. 1, plus additional layers. The tandem photovoltaic device 400comprises a first (“top”) cell 410 and a second (“bottom”) cell 420connected by a tunnel junction. In tandem thin-film solar cells,desirable characteristics for a TCO front-contact include: highband-gap, high transmission of incident light, high conductivity and lowresistivity.

The first (“top”) cell comprises a substrate layer 110, a firsttransparent conductive oxide (TCO) layer 120, an optional highresistivity transparent (HRT) layer 195 disposed on the first TCO layer120, an optional n-type first window layer 185, a p-type first absorberlayer 130. The tunnel junction between top cell 410 and bottom cell 420includes a heavily p+ doped interface layer 140 that forms the p+layerof a tunnel junction and a heavily n+ doped transparent contact layer150 that forms the n+layer of a tunnel junction. In some embodiments,the p+layer of the tunnel junction comprises the final p+ ZnTe layer ofthe top cell and the n+layer comprises a transparent conducting oxidesuch as zinc oxide doped with aluminum.

With continued reference to FIG. 9, the second (“bottom”) cell comprisesa second transparent conductive oxide (TCO) layer 220, a n-type secondwindow layer 285 (optional), a p-type second absorber layer 230, and ametal back contact 290. The metal back contact 290 may include gold,platinum, molybdenum, tungsten, tantalum, palladium, aluminum, chromium,nickel, or silver. The second cell receives the solar light transmittedthrough the first cell.

In one embodiment, the first cell utilizes CdSe_(x)Te_((1-x)) for thep-type first absorber layer 130 and the second cell utilizes a p-typesecond absorber 230 with a bandgap that is lower than the band-gap ofthe first absorber; for example a bandgap of about 1.4-2.2 eV in thefirst absorber and about 0.6 to 1.4 eV in the second absorber. Thesebandgaps can be selectively tuned up or down by the addition of dopantsusing methods known in the art. For example, Se can be used with CdTeabsorbers to tune a bandgap down while Zn can be used to tune a bandgapup. In addition, the layers comprising the n-p junction can be graded totune the bandgaps at the junction to significantly reduce surfacerecombination and increase absorption of the solar spectrum to improvepower conversion efficiency. In alternative embodiments, one may employan n-type absorber and the tunnel junction may be reversed in its orderor may be unnecessary.

The photovoltaic device 400 further includes electrical connections thatprovide a current path to communicate generated current flow, such asfrom one photovoltaic cell to adjacent cells in a module or from onephotovoltaic module to adjacent modules in an array. Alternatively, theelectrical connections may direct the current flow to an external loaddevice where the photo-generated current provides power.

In a monolithically integrated, series connected, two-terminal device,the second cell is electrically connected to the first cell through thep+/n+ tunnel junction.

In a stacked, parallel connected, four-terminal device, the tunneljunction transmits light to the lower cell and the n+ layer of thetunnel junction functions as a back contact for the top cell. In anembodiment, a first conductive lead is connected to a first TCO layer120; a second conductive lead is connected to the transparent contactlayer 150; a third conductive lead is connected to the second TCO layer220; and a fourth conductive lead is connected to the metal back contact290.

In embodiments of the invention, methods, apparatuses and/or structuresprovide for the following: device layers with energy band-gaps in therange between approximately 1.1 eV and 2.1 eV; optional compositionalgrading of heterojunction layers to optimize the structure and reduceinterface recombination; transparent back contact and tunnel junctionlayers suitable for bifacial and multi-junction devices; the productionof heavily doped material grown over a superstrate or substrate, insitu, near front and back contacts to create one or more ohmic contacts;reduced recombination of electrons and holes, and providing costeffective structures. In embodiments, the capabilities above, whencombined, allow for control over light absorption and charge carrierflow in the device for optimized performance. Examples of tuned tandemsolar cells producing high efficiencies are described. Methods andstructures of the present disclosure can provide photovoltaic deviceswith improved short circuit current (Jsc), open circuit voltage (Voc),and fill factor (FF) in relation to prior art thin film photovoltaicdevices.

The principle and mode of operation of this invention have beenexplained and illustrated in its preferred embodiments. However, it mustbe understood that this invention may be practiced otherwise than asspecifically explained and illustrated without departing from its scope.

1. A photovoltaic device comprising: an n-type layer selected from afirst transparent conductive oxide layer or a window layer, or both; ap-type absorber layer disposed on the n-type layer, wherein the absorberlayer consists essentially of CdSe_(x)Te_((1-x)), wherein x is fromabout 1 at. % to about 40 at. %; and a transparent tunnel junctiondisposed on the absorber layer, the tunnel junction comprising atransparent interface layer doped to be p+type, wherein the interfacelayer comprises Cd_(y)Zn_((1-y))Te, doped with copper (Cu), wherein ymay be from 0 to about 60 at. %, and a transparent contact layer dopedto be n+type, wherein the interface layer is disposed between theabsorber layer and the transparent contact layer, and wherein thetransparent contact layer comprises a second transparent conductiveoxide.
 2. The photovoltaic device of claim 1, wherein the amount of Sein the absorber layer is graded such that x varies from a minimum ofabout 1 at. % to a maximum in the range of about 15 at. % to about 30at. %.
 3. (canceled)
 4. The photovoltaic device of claim 1, wherein thetransparent contact layer is selected from the group of conductiveoxides consisting of fluorine-doped tin oxide, aluminum-doped zincoxide, cadmium stannate, and indium-doped tin oxide.
 5. The photovoltaicdevice of claim 4, wherein the transparent contact layer consistsessentially of ZnO and Al.
 6. The photovoltaic device of claim 5,wherein the concentration of aluminum (Al) is about 4 to 6 atomicpercent.
 7. The photovoltaic device of claim 1, wherein a thickness ofthe transparent contact layer is about 0.02 μm to about 1.0 μm.
 8. Thephotovoltaic device of claim 1, wherein the device further comprises asecondary conductor in contact with the transparent contact layer. 9-10.(canceled)
 11. The photovoltaic device of claim 1, further comprising anoptical reflector (ORF) layer disposed on the transparent contact layer.12. The photovoltaic device of claim 11, wherein the ORF layer comprisesa material selected from the group consisting of gold, silver, andaluminum.
 13. The photovoltaic device of claim 12, wherein the ORF layerhas a thickness in a range from 50 nm to 200 nm.
 14. The photovoltaicdevice of claim 1, further comprising an electron reflector layer (ERF)disposed between the absorber layer and the interface layer. 15-17.(canceled)
 18. A bifacial photovoltaic device comprising: asemiconductor layer stack defining a front surface and a back surface,the semiconductor stack including an n-type layer selected from atransparent conductive oxide layer or a window layer; and a p-typeabsorber layer disposed on the n-type layer, the p-type absorber layerconsisting essentially of CdSe_(x)Te_((1-x)) wherein x is from about 1at. % to about 40 at. %; a transparent tunnel junction comprising aninterface layer doped to be p+ type, and a transparent contact layerdoped to be n+ type, wherein the interface layer is disposed between theabsorber layer and the transparent contact layer, and wherein thetransparent contact layer comprises a transparent conductive oxide; ap-n junction, formed by the n-type transparent conductive oxide adjacentthe p-type semiconducting absorber layer; wherein the n-type transparentconductive oxide is substantially free of zinc; wherein the p-typesemiconducting absorber layer comprises cadmium and tellurium; anelectron reflector (ERF) layer disposed between the p-typesemiconducting absorber layer and the p+type interface layer; andwherein the p+type interface layer comprises zinc and tellurium; whereinthe n+type transparent conductive oxide layer comprises aluminum dopedzinc oxide; and wherein the n+type transparent conductive oxide layer isa back contact for the bifacial photovoltaic device. 19-22. (canceled)23. A photovoltaic device comprising: an n-type layer selected from afirst transparent conductive oxide layer or a window layer, or both,disposed on a substrate; a p-type absorber layer disposed on the n-typelayer, wherein the absorber layer consists essentially ofCdSe_(x)Te_((1-x)), wherein the amount of Se in the absorber layer isgraded such that x varies from a minimum of about 1 to a maximum in therange of about 15 at. % to 30 at. %; and a transparent tunnel junctiondisposed on the absorber layer, the tunnel junction comprising atransparent interface layer doped to be p+type, and a transparentcontact layer doped to be n+type, wherein the interface layer isdisposed between the absorber layer and the transparent contact layer,and wherein the transparent contact layer comprises a second transparentconductive oxide. 24-25. (canceled)
 26. The photovoltaic device of claim23, wherein the transparent contact layer is selected from the group ofconductive oxides consisting of fluorine-doped tin oxide, aluminum-dopedzinc oxide, cadmium stannate, and indium-doped tin oxide.
 27. Thephotovoltaic device of claim 26, wherein the transparent contact layerconsists essentially of ZnO and Al, wherein a concentration of aluminum(Al) is about 4 to 6 atomic percent.
 28. (canceled)
 29. The photovoltaicdevice of claim 23, wherein a thickness of the transparent contact layeris about 0.02 μm to about 1.0 μm. 30-33. (canceled)
 34. The photovoltaicdevice of claim 23, further comprising an optical reflector (ORF) layerdisposed on the transparent contact layer.
 35. The photovoltaic deviceof claim 34, wherein the ORF layer comprises a material selected fromthe group consisting of gold, silver, and aluminum.
 36. The photovoltaicdevice of claim 34, wherein the ORF layer has a thickness in a rangefrom 50 nm to 200 nm.
 37. The photovoltaic device of claim 23, furthercomprising an electron reflector layer (ERF) disposed between theabsorber layer and the interface layer. 38-44. (canceled)